A serum lactate dehydrogenase (LDH) test measures the total intracellular activity of this enzyme, which is found in almost all body tissues, particularly the heart, liver, muscles, kidneys, brain, and red blood cells. It is released into the bloodstream when there is damage or breakdown of the membranes of these cells. It is a sensitive but nonspecific marker of cellular damage and is often ordered as part of an initial evaluation or to monitor certain diseases.